Phosphorus catenation

Phosphorus (like C and S) exists in many allotropic modifications which reflect the variety of ways of achieving catenation. At least five crystalline polymorphs are known and there are also several amorphous or vitreous forms (see Fig. 12.3). All forms, however, melt to give the same liquid which consists of symmetrical P4 tetrahedral molecules, P-P 225 pm. The same molecular form exists in the gas phase (P-P 221pm), but at high temperatures (above 800°C) and low pressures P4 is in equilibrium with the diatomic form P=P (189.5 pm). At atmospheric pressure, dissociation of P4 into 2P2 reaches 50% at 1800°C and dissociation of P2 into 2P reaches 50% at 2800°. 

In the light of several allotropic modifications known for phosphorus, the relatively high single bond energies and the tendency of phosphorus to catenate it remains mysterious, and that apart from P4 and As4 only very scarce information on isolated E cluster molecules is available from hard experimental data. In contrast, a vast amount of solid theoretical work has been performed [11],... 

The direct linking of like atoms, such an essential feature of carbon chemistry, occurs to only a limited extent with other elements. Metal-metal bonds are not uncommon but they are always weak examples are found in the polynuclear carbonyls (p. 306) and compounds such as SugRg and PbgRfi (R = alkyl). Among the other non-metals catenation is displayed by boron, silicon, germanium, phosphorus and sulphur none of the substances... 

Transition metal phosphinidene complexes were originally prepared in order to access what was expected to be a rich chemistry of phosphorus(I)T However, terminal phosphinidenes have been found to be difficult to prepare, partly because they tend to either catenate or bridge. This is a manifestation of the double-bond rule for the main group elements. However, breakthroughs in syntheses of these complexes came about through the use of devious synthetic techniques in combination with sterically encumbered ligands. 

The review emphasized particular findings and included a review of phosphorus-containing polymers and catenation polymers. 

Elements such as phosphorus, silicon, tin, germanium and sulfur form catenated polymers similar to carbon, but such catenation does not usually lead to (homo) chains greater than... 

Both phosphorus and arsenic show a significant tendency to catenation, forming a series of cyclic compounds (RP) and (RAs) where n = 3-6, as well as some R2PPR2 and R2AsAsR2 compounds. 

Nitrogen often forms multiple bonds whereas the remaining pnictide elements have less tendency to do so. Phosphorus, in particular, has much greater tendency than nitrogen to catenate and form continuous chains of single bonds, that is, -P-P-P-P-. All pnictide elements except nitrogen (N) occur in more highly polymerised form. 

For p-block elements, phosphorus exhibits above-average catenating properties. Thus, although linear, single-bonded chains tend to be unstable, elemental phosphorus forms a variety of single-bonded cage structures. 

Recently some bicyclo and cage structures based on P—P bonds have been obtained by reacting a mixture of RPCl and PCI3 with magnesium. Only the phosphorus skeletons are shown (Fig. 4.12). These discoveries show that the chemistry of catenated phosphorus derivatives, like that of catenated silicon compounds, is much more diverse than was imagined even ten years ago. 

Complexes possessing (Fe2(C0)g units bridged by two phosphorus donors include CFe2(y-P R )(CO)g] containing catenated polyphosphorus ligands (R = Me, Ph) [fFe (CO)(y-P Me )], an organo-... 

That phosphorus exhibits extensive catenation within its allotropes has long been rationalised as a consequence of its periodic diagonal relationship to carbon [174, 175]. Although, as typical non-metal and semi-metals, respectively, both phosphorus... 

Catenation is defined as the self-linking of an element to form chains and rings. Carbon, then, given the above discussion, is the all-time champion catenator, much better than silicon (or sulfur, boron, phosphorus, germanium, and tin, the other elements that show this ability). Why should this be so A comparison of the relevant carbon and silicon bond energies as shown below is helpful ... 

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